Modelling Undrained Shear Strength and Pore Pressure Based on an Effective Stress Soil Model in Limit Equilibrium Method

This thesis presents the calculation method Hybrid su that is used to calculate the undrained shear strength or excess pore pressure in soft clay, based on effective strength parameters, in a limit equilibrium (LEM) framework. The Hybrid su method (HSU) can take into account the effects of anisotropy, consolidation state, volumetric hardening, and to some extent, effects of undrained loading rate. The calculation method is intended to be used as a fairly simple design tool with enough complexity to account for the most important properties of undrained soil behavior.Advanced finite element (FEM) soil models could be used to accurately model and calculate embankment stability. While such models can be very accurate, they are demanding in terms of user expertise, parameter determination and time. On the other hand, basic stability design work is typically carried out with limit equilibrium methods (LEM), where strength input often comes from field vane testing (φ = 0 analyses). The largest inaccuracy in φ = 0 analyses is often in strength determination. In the case of undrained effective stress calculations (c’-φ’ analyses), the inaccuracy lies in determining excess pore pressure.The HSU method provides improvements in the determination of su and Δu in the context of LEM. It is based on the anisotropic critical state soil model S-CLAY1. The formulation of the original model is simplified with reasonable assumptions to obtain a closed form solution for undrained shear strength, based on effective strength parameters. The method can also be used to derive excess pore pressure at the failure state. As the method relates the calculated pore pressure and effective stresses to the assumed failure state, the known property of overestimating shear strength in traditional undrained effective stress analyses is effectively solved.The method is validated by fitting it to laboratory data on various soft clays, as well as back-calculations of several failed embankments. HSU is shown to give good results with very reasonable parameter combinations. The recommended use of the HSU method is determining su, which is then used as input in a typical total stress limit equilibrium analysis. The proposed approaches of determining Δu for undrained effective stress analyses work fairly well. However, the Δu approach is mainly presented as a proof of concept due to its complex and redundant nature compared to the su approach.